The explosive growth of digital communications technology has resulted in an ever-increasing demand for bandwidth for communicating digital information, such as data, audio and/or video information. To keep pace with the increasing bandwidth demands, new or improved network components and technologies must constantly be developed to perform effectively at the ever-increasing data rates. In optical communication systems, however, the cost of deploying improved optical components becomes prohibitively expensive at such higher data rates. For example, it is estimated that the cost of deploying a 40 Gbps optical communication system would exceed the cost of existing 10 Gbps optical communication systems by a factor of ten. Meanwhile, the achievable throughput increases only by a factor of four.
Thus, much of the research in the area of optical communications has attempted to obtain higher throughput from existing optical technologies. In conventional optical transmission systems, dispersion compensation typically requires expensive optical compensators. In order to relax the need for dispersion compensating modules (DCM), duobinary pulse shaping techniques have been considered for optical channels. Duobinary pulse shaping techniques reduce the required signal bandwidth by a factor of two by mapping a binary data signal to a three-level signal.
Typically, duobinary modulation formats employ two signal levels (+1 and −1) to represent a binary value of one, and one signal level (0) to represent a binary value zero. The narrow spectral width of duobinary signals, compared to a conventional non-return-to-zero (NRZ) binary signal, has been exploited to minimize dispersion sensitivity and to enable dense wavelength division multiplexing techniques. It has been observed that modulating an optical signal with a duobinary electrical signal, as opposed to a standard non-return to zero (NRZ) signal, reduces the optical spectrum by a factor of two.
A need exists for an optical communication system that employs duobinary pulse shaping techniques to provide further improvements in spectral efficiency. Among other benefits, improved spectral efficiency will allow greater tolerance to dispersion and the use of generic and available optical technologies. A factor of four improvement in spectral efficiency, for example, would provide a 40 Gbps optical communication system using existing 10 Gbps optical communication systems.